Tires, etc

ABSTRACT

The inner surface of a green tire provided with an air-sealing liner, is cured or partially cured to produce a steam- and water-resistant surface, and the tire after such treatment is cured in a usual press without an air-bag or bladder. 
     One or more plies of a tire, usually the outer fabric plies, are partially cured before assembly in the tire, to insure that plies so located are cured, without over-curing inner and outer portions of the tire. 
     New tires are made by placing a carcass, whether green or partially cured, in a mold and extruding rubber over the outer surface of this carcass to fill the mold, and then giving the final cure to the resulting tire. 
     Tires are retreaded by applying ribbons of tread stock over the carcass prepared in any conventional manner, and then completing the cure without the use of a mold. 
     Retreaded tires are cured by placing a carcass prepared in any conventional manner, in a mold, extruding tread stock into the mold around the carcass, and then curing. 
     A tire, the inner surface of which is provided with a non-cured liner reinforced with a non-wicking fabric is cured without an air-bag or bladder. 
     Curing or partial curing may be effected by electron irradiation. Progressive irradiation is disclosed by which an object is repeatedly irradiated to attain a desired cure. Other methods of curing may be preferable in some operations.

This application is a continuation of application Ser. No. 321421, filedJan. 5, 1973, which is a continuation-in-part of application Ser. No.838,512 filed July 2, 1969. Both applications have been abandoned.

The invention relates more particularly to the use of electron radiationin the production of new tires and the retreading of old tires, althoughother means of radiation may be employed, and for most operations othermeans of curing may be used, as explained.

Electron radiation has been known for some years and in recent yearsequipment utilizing up to a million volts or more has become availablefor commercial use. The voltage employed determines the depth ofpenetration of the electron rays. For example, radiation generated by amillion volts will penetrate an item having a density of 1 to a depth of1/8 inch. By varying the voltage, the depth of the penetration may bevaried. Thus, tires may be cured to a depth 1/8 inch or more, dependingupon the rubber employed, the compounding formula, and the voltage used.By using higher voltages and radiating both surfaces of a tire, tires ofmuch greater thickness may be cured. When only one surface isirradiated, beyond the depth at which the rubber is cured there is apartial curing which becomes less as the distance from the rubbersurface increases.

The amperage will be varied depending upon the area subjected toradiation. The new equipment provides for irradiating an area of severalsquare inches, up to, for example, 12 square inches or more.

By irradiating natural rubber or synthetics such as butadiene-styrene,polybutadiene, polyisoprene, etc., the molecular structure isrearranged. Irradiation has no beneficial effect on so-called "butyl"(copolymer of isobutene and either isoprene or butadiene) rubber. Therearranged rubber is partially or completely cured. By that it is meantthat rubber which is thermoplastic when irradiated becomes partially orentirely thermoset (vulcanized) with or without sulfur.

Radiation may be used to treat a green tire or part of a green tire, orthe tread in retreading, or plies used as a liner for a green tire ortread produced from ribbons of tread stock in retreading or the extrudedtread stock in the manufacture of new tires or in retreading.

The rubber may be cured instantaneously by subjection to irradiation,and this may be done in a continuous operation in which the source ofradiation and the rubber are moved continuously relatively to oneanother. Alternatively, the rubber may be cured by progressiveirradiation. Progressive irradiation is accomplished by partially curinga piece of rubber in one exposure to the source of radiation, and thengiving it one or more additional exposures each of which partially curesthe rubber, until the rubber is cured. For example, by exposing the samerubber a number of times as by rotating a tire continuously and applyinga thin layer of the rubber to the tire as it is rotated, and continuingthe application of the thin layer throughout several rotations of thetire and as each thin layer is applied to the tire, exposing the rubberto irradiation to partially cure it so that as the tire is rotated anumber of times in building up a desired thickness of the carcass ortread and by continuing the rotation a number of times after completingthe application of the rubber, the rubber is progressively andcompletely cured.

The production of new tires will be discussed first, and in thisconnection reference will be made to the following figures;

FIG. 1 is a section through a green tire with a liner with the innersurface being irradiated;

FIG. 2 is a section of a small portion of a green tire with a linerafter the inner surface has been irradiated;

FIG. 3 is a section of a small portion of a tire provided with apre-cured liner;

FIG. 4 is a section through a mold containing a tire being cured withoutan air-bag or bladder, the inner surface of the liner being shown asthat adapted to be contacted by the tire-inflating fluid and the outersurface of the liner being shown as that which is adapted to contact thetire carcass;

FIGS. 5 and 6 are enlarged details of a portion of a modification of thetire and mold shown in FIG. 4;

FIG. 7 is a section through a conventional mold containing a tire beingcured without an air-bag or bladder;

FIG. 8 is a section through a conventional mold provided with means forextrusion of tread rubber into the mold; and

FIG. 9 is a section of a tire before curing, with several outer fabricplies pre-cured.

Then the retreading of tires will be discussed, and reference will behad to the following figures;

FIG. 10 is an elevation of a stand with a tire being retreaded on it,with the tread stock being extruded, and means for shaping the tread;

FIG. 11 is a section on the line 11--11 of FIG. 10 showing a roller thatmay be used for shaping the tread in position on a section through atire being retreaded;

FIG. 12 is a section through a modified roller; and

FIG. 13 is a section through the liner for the tubeless tire claimedherein.

NEW TIRES Tires Cured Without an Air-Bag or Bladder

It has long been the wish of the industry to avoid the use of an air-bagor bladder in curing tires. The bag or bladder separates the innersurface of the green tire in a mold from the steam or hot water used toapply pressure to the bag or bladder to force the tire against the innersurface of the mold and thus remove air from the green tire and mold andsubsequently cure the tire.

The invention relates to methods of curing the tire without an air-bagor bladder, and to methods which employ an air-bag or bladder.

According to one method, a green tire is formed in the usual manner,with a liner. Then this green tire is subjected to electron irradiationto cure the exposed inner surface or stratum to a depth of perhaps 1/16inch or more, with partial curing at a greater depth; or only partiallycuring the exposed inner surface to a depth of perhaps 1/16 of an inchor more.

According to another method, a liner (such as is used in the productionof tubeless tires) is partially cured or completely cured, and then thisliner is used as the inner ply in building up a green tire. FIG. 13 is amodification of a liner such as is used commercially in the productionof tubeless tires.

Liners to be cured or partially cured by irradiation must haveincorporated in them sufficient natural rubber or synthetics whichreacts to irradiation, to accomplish the degree of cure desired. If theliner is composed of butyl rubber or other rubber which is not cured byirradiation, it may be laminated to natural rubber or other rubber whichcan be so cured. This lamination would preferably be on the innersurface. Any of the plies may incorporate a reinforcing element, forexample, non-wicking fabric. This ply of natural rubber or synthetic maybe cured or partially cured by electron irradiation before the tire iscured and acts as a sealing element during the curing of the tirewithout an air-bag or bladder. The butyl ply is cured when the tire iscured in any conventional manner. Any of the plies may incorporate areinforcing element.

Whether the liner is cured before or after being built into the tire, itis incorporated in the tire by joining its two narrower ends, usually bya beveled splice or by butting the two ends together to produce a band.To assure a good seal it may be advantageous to place a locking strip,such as a strip of uncured or partially cured stock over the jointand/or a ply of non-wicking fabric, such as is used in conventionaltire-bead construction. This fabric may be incorporated in the strip asa stabilizer. The locking strip may be cured or partially cured as byirradiation or any other process, before the tire is placed in a moldand cured.

The inner surface of a green tire with a liner, whether the liner becured before incorporation in the tire or whether the inner surface ofthe liner be cured after the green tire is formed, is resistant to thepassage of steam or hot water used for inflation of the tire duringmolding. Therefore, this green tire can be cured in a mold without anair-bag or bladder. The mold may be of usual construction except thatthere is no provision for an air-bag, and modification for supportand/or formation of the beads may be desirable.

FIG. 1 is an illustration of equipment that may be used for curing theliner of a green tire in which an uncured liner has been incorported onthe tire-building drum. The green tire 5, provided with the liner 6, isplaced over a roller 8 the shaft 9 of which will ordinarily bemechanically driven, although it may be operated by hand. The green tireis readily placed over the roller as indicated in FIG. 1. Although theinner surface of the tire, and primarily the liner, may be cured by anymeans, it is preferably cured by electron irradiation. A suitable device12 for effecting irradiation is moved into the position shown in FIG. 1to effect the irradiation. Normally, a longitudinal section of 12,extending through approximately the entire width of the tire, will giveoff rays, and the entire inner surface of the tire will be irradiated bymoving that surface under the longitudinal section by rotating theroller 8. Normally the irradiation will not be sufficient to cure anymore than the inner surface of the liner to a depth of perhaps 1/16inch. It may, however, cure the liner. FIG. 2 illustrates how, when theirradiation is controlled to cure only the inner surface of the liner6', there is partial curing beyond the depth that is "firm cured". By"firm cured" we refer to sufficient cure to prevent steam or waterpenetrating the tire when subjected to curing in a mold. The outersurface of the liner may be partially cured or not cured.

In the alternative method, the liner is cured or partially cured priorto assembly with the fabric plies. If usual methods of curing areutilized, the entire liner will normally be completely cured and it maybe desirable to use an adhesive when assembling with the carcass. If theliner is cured by electron irradiation, the cure generally will only bea surface cure of perhaps 1/16 inch more or less. FIG. 3 illustrates agreen tire, the liner of which has been partially cured by irradiationbefore assembly with the carcass. The inner surface of the liner hasbeen completely cured, the opposite surface of the liner 6" is partiallycured or may not be cured.

Thus, according to this invention, the liner may be pre-cured,completely or only partially, before incorporation in the tire, or itmay be completely or only partially cured after being incroporated inthe tire.

A reinforcing element such as a sheet of fabric made of non-wickingmaterial, such as nylon monofilament, or a non-wicking fabric, such asis used in conventional tire-bead construction, may be incorporated onthe inner surface or outer surface of the liner, or in between thesesurfaces. This liner will usually be composed of butyl rubber. A sheetof such fabric may be coated with natural or synthetic rubber in aconventional manner and be assembled with the tire adjacent to theliner, which may be made of butyl rubber, and preferably on the innersurface of the liner. The reinforcing element prevents moisture fromgetting into the carcass. The tire is built in a conventional manner andmay be formed and/or cured in a mold without an air-bag or bladder.

FIG. 4 illustrates a mold for curing any of these tires, and it will benoted that no air-bag or bladder is utilized. The mold is constructed inany usual manner. It is shown as being formed of an upper part 15 andlower part 16 with two bead rings 17 fitted into the upper and lowerparts to contact the beads of the tire 5. The liner 6 has beenpre-treated or prepared in any of the ways previously discussed, so asto render it impervious to steam and hot water. The bead-sealing ring 18may be inserted in the same operation that the conventional bladder orair-bag is inserted.

The ring 18 may be a bull ring made of steel or other metal, or it maybe made of rubber or other hard elastomeric material. This ring may besegmented. The ring is placed between the beads of the tire before themold is closed, and serves as a gasket.

To prevent leakage between the inner surface of the tire and this ring18 when liquid pressure is generated in the tire, it may be desirable toprovide irregularities in that portion of the surface of the ring 18which is contacted by the tire at or above the beads, or both. Theseirregularities may be in the nature of projections 20 as shown on anenlarged scale in FIG. 5, or grooves 21 shown on an enlarged scale inFIG. 6. These run longitudinally around the circumference of the ring18. The steam or hot water is led into the mold through the pipe 19 andprovides sufficient pressure within the tire to force it against themold to groove the tread and form any desired identification and indiciamarks desired on the tire surface. In FIG. 7 the ring 28 is formed withspring extensions 29. Before the mold is closed these are in theposition shown in dotted lines. As the mold is closed these extensionsare pressed inwardly by the tire and shape the bead and form a tightseal. This ring 28 may be a steel ring or it may be of a firm rubber orresinous material. If made of material other than steel, the extensions29 may be much thicker and the ring may be solid up to these extensions29 and be compressible. Various devices may be employed to permitdeflection of the outer surface of the member 29 from the position shownin dotted lines, to that shown in full lines.

The tire will be cured at usual temperatures, and steam or hot water isused in the usual manner, but without an air-bag or bladder. Air-bagsand bladders are short-lived. By eliminating them, not only is theoriginal cost eliminated but also the labor and other cost of frequentreplacement are eliminated.

A new method of applying a tread to a green carcass is illustrated inFIG. 8. The carcass 20 is built up in a conventional manner, with orwithout liner 21, on a tire-building machine and the carcass 20 isplaced in the mold shown in FIG. 8, which may be heated in theconventional manner, which comprises a lower part 22 and an upper part23 with two bead rings 24 fitted into the upper and lower parts tocontact the beads of the tire 20. The mold is conventional except forthe orifice 26 through which tread rubber is injected, being fed fromnozzle 27. A conventional air-bag or bladder may be used or the processusing neither of these and previously described may be used. Hot wateror steam under pressure is used to expand the green carcass against thesides of the mold. This pressure, before the tread is injected into themold, may be less than conventional curing pressures and only sufficientto press the green carcass into position against the sides of the mold.The tread stock is injected at temperatures at which it flows readilyunder pressure, such as temperatures of approximately 280°F. and higher,through orifice 26 from nozzle 27.

The tread stock is usually introduced at or near the low point of theempty portion of the mold cavity outside of the carcass. It may beintroduced at several points. As it is introduced, the air is ventedthrough the air vents usually found in the tire mold, or separate ventmeans may be provided. When the tread has been injected the pressure inthe tire is raised to conventional levels and the carcass is completelycured. It is impractical to thus inject tread stock which contains theusual quantity of sulfur and accelerator or the like for curing becausethe temperature required for rendering the rubber sufficiently fluid forinjection is high enough to cure such stock. An exception is, whencold-feed extruders are used, in which case cold stock is fed into themachine and is raised to high temperatures only for a small increment oftime before being extruded. According to this invention, the curingagent added to the tread rubber is only sufficient to partially cure thetread so that it may be removed from the mold after curing the carcass,without causing damage to the tread, such as blowing. The tire is thenremoved from the mold and the tread is cured by rearrangement of themolecules of the rubber in the tread stock, by irradiation or othermeans. An advantage of this method is the elimination of inventories ofuncured treads with inherent savings such as floor space, investment,labor, etc.

Alternatively, if irradiation which penetrates to a sufficient depth isused, the tread may be cured in the mold before the tire is removed.

When tires are cured by irradiation, even though no sulfur be employed,reinforcing agents, antioxidants, antiozonants, etc. may advantageouslybe added.

Pre-Curing Certain Carcass Plies

It is common knowledge in the industry that when very thick tires arecured in a mold in the ordinary manner, either (1) outer carcass pliesare only partially cured or (2) inner carcass plies are over-cured whenthe heat for curing is supplied internally by steam or hot water throughan air-bag or bladder and additional steam or other means is providedfor heating the mold. It is proposed to overcome this difficulty andshorten the curing time by pre-curing certain of the plies--eithercompletely or preferably only partially. Ordinarily these will beseveral of the outer fabric plies, because the difficulty experiencedhas been encountered in attempting to insure the curing of these outerplies at the interface between the fabric plies and the tread, andusually in the outer fabric plies.

Other Methods of Manufacturing New Tires

FIG. 9 shows a cross-section of a tire, with eight fabric plies 30 andtread 31, the carcass of which may be built in the usual manner. Theportion of the carcass under the tread 31, prior to the assembly oftread 31, is subjected to electron irradiation and one or more of theplies may be partially cured or cured. The building of the tires is thencompleted in the usual manner. During the curing operation in the moldit will take less "time temperature" to cure such a tire because theplies under the tread have been pre-cured, than in the conventionalmethod. Therefore, the result is a more uniformly cured tire. Also ashorter curing time is required in the mold which lowers cost. Anadhesive may be employed between the carcass 30 and the tread 31.

Difficulty is experienced in uniformly curing the thickest portion ofvery large tires -- that is, tires which comprise many plies, such asfifteen or twenty plies or more. The time required is also objectionablygreat. It is the section of the tire through the tread which is thickestand therefore most difficult to cure. To overcome this, it is proposedto precure, partially or completely, certain of the plies before theassembly of the tire is completed. It is not necessary to precure any ofthe inner plies because they are readily cured by heat from the steam orhot water used to inflate the tire in the mold. Thus, for example, ifthere are 20 plies in the tire, it is not necessary to pre-cure any ofthe first five plies. Thus the first 10 plies may be built on the drumin the usual manner. Then the outer surface of this partially builtcarcass, and preferably the center portion which is to come under thetread, is irradiated to partially cure the five outer plies. Thenseveral more plies, for example five more, are built up in the usualmanner and these are similarly cured. The last five are then applied andthese are similarly treated, except, as the tire is enlarged, it may bedesirable to further cure the outer plies than those which are locatednear the inside of the tire. If the tire is very large it may bedesirable to vary the amounts of cure in the various segments as theyare applied so that those plies which are farthest from the surfaces ofthe completed tire are cured to a greater degree than those nearer thesurface. If the tread is very thin, the plies immediately under it willnot be pre-cured to the same extent as those farther from the surface ofthe tread.

A desirable method of pre-curing is by progressive irradiation. This isaccomplished by placing over the building drum a radiation device 12which covers only the tread portion of the tire, as shown in FIG. 1.During the progressive irradiating operation the number of pliespre-cured in each group may vary and the number of groups may vary aswell as the number of plies that are to be irradiated, depending on thethickness of the plies, compounding, voltage used, etc. The number oftimes the tire is rotated to pre-cure the different groups of plies maybe varied to control the degree of cure in each group. This progressivecuring with irradiation assures a more uniformly cured tire and ashorter curing time which will lower the cost.

In building a tire, certain of the plies may be precured before assemblyon the drum. FIG. 9 shows a cross-section of a tire with eight fabricplies 30 and tread 31. It may be built in by the following method. Aconventional tire-building machine is used and the first, second, third,fourth and fifth plies, for example, are assembled in the usual mannerand are uncured. Then the sixth, seventh and eighth plies which havebeen partially or completely pre-cured are assembled into the tire.These cured or partially cured plies are so thin that they can be shapedas desired. Then the tread is added. If these cured plies are pre-curedby electron irradiation it is not necessary that they contain anysulfur, but they must be completely cured unless the subsequent curingof the tire is accomplished by irradiation. By only partially curingcertain plies, adhesion between the partially cured and cured plies isimproved. Adhesive may be employed to insure adhesion.

When a tire is built with many plies in the carcass, such as 20, it maybe advantageous to use a combination of methods disclosed in thesespecifications. The conventional tire-building machine may be used. Alsothe conventional method of building a tire with the incorporation ofirradiating and the use of pre-cured and/or partially pre-cured pliesmay be used. For example, the first, second, third, fourth and fifthplies are uncured, and are assembled in the usual manner. The next tenplies which are the sixth through the fifteenth, are partiallypre-cured, as by irradiation, and assembled in the usual manner. Plies16, 17, 18, 19 and 20 are uncured, and assembled in the usual manner.Then that portion of these last five plies which will be under the treadafter it is assembled is irradiated and partially cured. The tread isthen assembled and the tire is cured and finished in the usual manner.

This method assures a more uniform cure than the conventional method andshortens the cure time which saves cost.

RETREADING TIRES

Various ways are disclosed for retreading tires. In each case thecarcass will be prepared for retreading in the usual manner by removingunwanted tread and, depending upon the shape of the retread which is tobe applied, a part of the sidewall stock may be removed, all of which iscustomary in the different methods of retreading, using differentequipment.

FIG. 8 shows a mold for extruding tread into position on the uncured orcured carcass of a new tire. The equipment of FIG. 8 and the generalmethod there disclosed may be used for applying the tread to a carcasswhich is to be retreaded; this operation taking place after the carcasshas been prepared in the usual manner. It may be desirable to use air atroom temperature or cooler, and water at tap temperature or cooler inthe tire to create pressure in the tire. It may also be desirable to sodesign the mold that the heating cavity is only approximately over thetread which is being applied.

The advantages of this method of retreading tires are elimination oftread building facilities and the inventories of prepared uncured treadsall of which reduce costs. Also the carcass is subjected to less heatwhich has a deteriorating effect.

Instead of extruding the tread into a mold, the tread may be extrudedonto a prepared carcass by rotating the carcass and using a suitablyshaped nozzle that covers the width of the tread, or a narrower nozzle,the stock being reciprocated back and forth over the surface of thecarcass to build up a retread. Suitable apparatus is illustrated in FIG.10. The extruded rubber will ordinarily be heated to approximately280°F. or lower or higher to made it sufficiently plastic to be used asdescribed. It may or may not contain sulfur and/or curing agents and/oraccelerators. Some volatile solvent may be added to the rubber whichwill be evaporated before the curing of the tread is commenced, althoughusually this will be avoided because of the danger of solvent vaporforming pockets in the tread.

The methods illustrated in FIGS. 10 and 11 are particularly designed forthe retreading of airplane and truck tires which have a tread which isonly grooved, but may be used for any tires. FIG. 10 shows stand 50 withaxle 51 supporting wheel 52 with rim 53 on which the tire carcass 54 ismounted. A valve 56 in the rim through which the tire may be inflatedgives it some rigidity during the application of the tread. The tire isprovided with a scuff ring 57, but it is to be understood that tires ofany shape and design may be retreaded by the method disclosed. Asillustrated in FIG. 10, stock 58, sufficient to form the tread, isextruded from extruder 60 through nozzle 61. A forming tool 65 ispositioned on the supporting means 66 to shape the tread as the treadstock is extruded; and if desired, forming tool 65 may exert somepressure to force the formed tread against the carcass. There may beadditional forming tools, such as 65, at other points on thecircumference of the tread for the purpose of shaping and/or pressure.After the tread is applied the tire may be rotated several times for thepurpose of shaping and/or pressure.

The tread may be extruded with the proper tread outline andtread-forming tool 65 may be positioned on the supporting means 66 tomaintain the proper shape and/or exert pressure on the tread to press itagainst the carcass. There may be other forming tools, such as 65, atother points on the circumference of the tire.

Another method of operating is to extrude the tread stock as a thinribbon the width of the tread, while the tire is being rotated, andafter quite a few rotations sufficient tread will be applied to build upthe desired tread thickness. As the ribbons are being applied to thetire, the tread-forming roller will shape them to the proper design. Aroller 70, such as illustrated in FIG. 12, may be used. The carcass 67is shown with tread 68 built upon it.

Another method of operating is to extrude a tread stock as a thin,narrow strip while the tire is being rotated. The strip is oscillatedback and forth across the tread portion and after quite a few rotationssufficient tread will be applied to build up the desired treadthickness. As the ribbon is being applied to the tire the tread-formingtool 65 or tools shape it to the proper design. A tire retreaded inmanners just disclosed, is preferably cured by electron irradiationwhich may be accomplished by rotating tire and wheel 54 and 52,respectively, under a radiation device such as device 12 shown in FIG.1; and, if desired, may be cured in a conventional manner or acombination of both. A grooved tread design may be cut into the tread byany conventional method or a groove may be rolled into the green treadby forming tool as shown in FIG. 12, preferably after the tread has beenbuilt up or while the last layer is being laid on the tread.

A method which is preferred is illustrated by FIG. 10. Stock 58, being athin ribbon, for example, approximately 1/8 inch thick, andapproximately the width of the tread, is extruded as the tire is rotatedand the tread is built up to any desired thickness. Forming tool 65shapes the tread and several other forming tools may be used. Radiationdevice 59, similar to or identical with device 12 in FIG. 1, isillustrated in FIG. 10. It only partially cures the tread stock duringeach exposure as the stock is passed by it. The cure may, for example,be only a one-fourth cure. The radiation device may be adjusted to cureto a depth of 1/2 inch, more or less. At the completion of each rotationthe surface stock is only partially cured and, therefore, the next layerof hot stock adheres to it. The stock is progressively vulcanized as therotation continues and the successive layers are adhered to one another.

If the stock is one-quarter cured on each exposure, at the completion ofthe fourth rotation the first layer is fully cured, the second layer isthree-fourths cured, the third layer is one-half cured, and the fourthlayer is only one-fourth cured. This "progressive irradiation" continuesuntil the last layer is applied and then the tire is given threeadditional rotations which cures the last three layers.

A ribbon not the full width of the tread may be used and oscillated backand forth across the tread surface until the desired tread thickness isobtained, the level of radiation being adjusted to obtain the propercure.

Grooves in the tread may be made by the use of a forming tool such asthe tool 70 shown in FIG. 12.

In each of the retreading operations in which the tread stock is curedby irradiation, this stock may include sulfur and other curingingredients, or it may contain no such curing compositions becauseirradiation rearranges the molecular structure of the rubber and thuscures it. A small amount of sulfur may be included in such stock, andthe extrusion will take place rapidly before the rubber becomesobjectionably cured at the extrusion temperature. The amount of curingingredients that can be incorporated in the stock will depend upon theextruding temperature and the length of time that the stock ismaintained at this temperature. It is desirable to include a smallamount of sulfur and accelerator so that the stock will cure to a slightextent after it has left the extrusion device and been shaped on thecarcass. Such setting of the retreading stock will facilitate handlingthe retreaded tire before the cure is completed. If a normal amount ofsulfur, etc. are included in the rubber, unless the time during whichthe stock is heated is very short, curing will set in and the rubberwill become pre-cured before it is shaped on the carcass. An exceptionis, when cold-feed extruders are used, in which case cold stock is fedinto the machine and is raised to high temperatures only for a smallincrement of time before being extruded.

The advantages of these methods of retreading include not heating thecarcass in a mold, and, therefore, the life of the tire is increased andthe tire may be retreaded a greater number of times, the curing time isshorter, press equipment is eliminated and inventories of green treadstocks are eliminated and thus savings in cost are made.

Apparatus and processes for forming treads and curing them by electronirradiation have been disclosed. These tread stocks may be of ordinaryrubbers (except those which cannot be cured by electron irradiation,such as butyl rubber), and it has been explained that no sulfur isrequired for curing by irradiation. A desirable tread stock is a highmolecular weight butadiene-styrene copolymer such as has been founddesirable for tread stocks, but which is of high viscosity and difficultto handle in ordinary processing equipment. Reinforcing agents,anitioxidants, antiozonants, etc. may be included even though no sulfuris added.

I claim:
 1. The method of making a liner for a pneumatic tire, whichliner is composed of a plurality of rubber layers, which methodcomprises curing or partially curing the surface and the underlyingstratum of a layer of the liner without curing the inner layer of theliner which layer is a butyl rubber and which is adapted to face towardsubsequent tire-inflating fluid, and without completely curing thesurface of the liner which is adapted to contact the tire carcass andlaminating the layers, the uncured inner layer and the surface which isadapted to contact the tire carcass being otherwise suitable to bejoined by a good seal when the ends of the liner are united.
 2. Themethod of claim 1 in which the surface which is adapted to contact thecarcass of the tire is not cured.
 3. The method of claim 1 in which both(1) the surface which is adapted to contact the carcass of the tire and(2) the inner layer are cured when the tire is cured.